RPR112378 and RPR115781: two representatives of a new family of microtubule assembly inhibitors

Allocolchicinoids bearing a Michael acceptor fragment for possible irreversible binding of tubulin

We describe an attempt to apply the concept of covalent binding towards the highly active allocolchicinoids selected on the basis of SAR analysis of previously synthesized molecules. To achieve the irreversible binding of the agent to the cysteine residues of the colchicine site of tubulin protein, we synthesized a number of new allocolchicinoids bearing the acceptor moiety. Some of the new derivatives possess cytotoxic activity against COLO-357, BxPC-3, HaCaT, and HEK293 cell lines in a low nanomolar range of concentrations. A substoichiometric mode of microtubule assembly inhibition was demonstrated. The most active compounds possess close to colchicine general toxicity on mice.

Bifidenone: Structure-Activity Relationship and Advanced Preclinical Candidate

Bifidenone is a novel natural tubulin polymerization inhibitor that exhibits antiproliferative activity against a range of human cancer cell lines, making it an attractive candidate for development. A synthetic route was previously developed to alleviate supply constraints arising from its isolation in microgram quantities from a Gabonese tree. Using that previously published route, we present here 42 analogues that were synthesized to examine the structure-activity relationship of bifidenone derivatives. In addition to in vitro cytotoxicity data, data from murine xenograft and pharmacokinetic studies were used to evaluate the analogues. Compounds 45b and 46b were found to demonstrate promising efficacy in murine xenograft experiments, and 46b had significantly more potent in vitro antiproliferative activity against taxane-resistant cell lines compared to that of paclitaxel.

Tubulin inhibitors targeting the colchicine binding site: a perspective of privileged structures

The vital roles of microtubule in mitosis and cell division make it an attractive target for antitumor therapy. Colchicine binding site of tubulin is one of the most important pockets that have been focused on to design tubulin-destabilizing agents. Over the past few years, a large number of colchicine binding site inhibitors (CBSIs) have been developed inspired by natural products or synthetic origins, and many moieties frequently used in these CBSIs are structurally in common. In this review, we will classify the CBSIs into classical CBSIs and nonclassical CBSIs according to their spatial conformations and binding modes with tubulin, and highlight the privileged structures from these CBSIs in the development of tubulin inhibitors targeting the colchicine binding site.

Natural antitubulin agents: importance of 3,4,5-trimethoxyphenyl fragment

Microtubules are polar cytoskeletal filaments assembled from head-to-tail and comprised of lateral associations of α/β-tubulin heterodimers that play key role in various cellular processes. Because of their vital role in mitosis and various other cellular processes, microtubules have been attractive targets for several disease conditions and especially for cancer. Antitubulin is the most successful class of antimitotic agents in cancer chemotherapeutics. The target recognition of antimitotic agents as a ligand is not much explored so far. However, 3,4,5-trimethoxyphenyl fragment has been much highlighted and discussed in such type of interactions. In this review, some of the most important naturally occurring antimitotic agents and their interactions with microtubules are discussed with a special emphasis on the role of 3,4,5-trimethoxyphenyl unit. At last, some emerging naturally occurring antimitotic agents have also been tabulated.

Curcumin binds tubulin, induces mitotic catastrophe, and impedes normal endothelial cell proliferation

Curcumin, a component of turmeric spice that imparts flavor and color to curry, is thought to possess anti-inflammatory and antioxidant properties in biological tissues. However, while such efficacies have been described in the context of carcinogenesis, the impact of curcumin on normal cell cycle regulation is poorly understood. Here, we provide evidence of curcumin toxicity in proliferating bovine aortic endothelial cells, at concentrations relevant to the diet and below those previously reported in cancer models. Upon confirming curcumin's ability to upregulate hemeoxygenase-1 in a dose-dependent fashion, we found the minimally efficacious curcumin concentration to also inhibit endothelial cell DNA synthesis. Moreover, curcumin concentrations below the minimum 2 μM threshold required to induce hemeoxygenase-1 bound tubulin protein in vitro and triggered hallmark evidence of mitotic catastrophe in vivo. Concentrations as low as 0.1 μM curcumin led to disproportionate DNA segregation, karyorrhexis, and micronucleation in proliferating endothelial cells. While suggesting a mechanism by which physiological curcumin concentrations inhibit cell cycle progression, these findings describe heretofore unappreciated curcumin toxicity with potential implications for endothelial growth, development, and tissue healing.

Enantioselective Total Synthesis of Otteliones A and B, Novel and Powerful Antitumor Agents from the Freshwater Plant Ottelia alismoides

Otteliones A and B isolated from the freshwater plant Ottelia alismoides have attracted significant attention because of their potential as novel anticancer agents. In this review four independent enantioselective total syntheses and one formal synthesis of these natural products are presented with particular focus on their methodology and strategy.

New Diarylheptanoids and a Hydroxylated Ottelione from Ottelia alismoides

Ten new diarylheptanoids (2, 3, 4, 5a-d, 6, 7, and 8) have been isolated from an extract of Ottelia alismoides. The structures of these previously unknown metabolites were determined by NMR spectroscopic analysis. A previously unknown, hydroxylated analog of the known otteliones A and B (1a and 1b)–namely, 3a-hydroxyottelione (13)–was also isolated. The 1H NMR analysis of the Mosher esters of alcohols derived from otteliones A and B (S-17/R-17 and S-20/R-20) are also reported.

Synthesis and antiproliferative activities of ottelione a analogues

Through the syntheses of its C-1 desvinyl, C-7 methylene, C-7 exocyclic ethylidene, and various C-3 phenylmethyl analogues, the structure-activity relationship of antimitotic ottelione A (4) against tubulin and various cancer cells was established. The results indicated that compound 4 was a colchicine-competitive inhibitor and that the C-1 vinyl group is unnecessary for its potency, whereas the C-7 exocyclic double bond is essential, possibly because of its irreversible interaction with tubulin. Further optimization of the substituents on the phenylmethyl group at the C-3 position generated compound 10g with a 3'-fluoro-4'-methoxyphenylmethyl substituent, which was 6-38-fold more active against MCF-7, NCI-H460, and COLO205 cancer cells relative to 4. Results from in vitro tubulin polymerization assay confirmed the potency of compounds 4, 10g, and 11a.

An overview of tubulin inhibitors that interact with the colchicine binding site

Tubulin dynamics is a promising target for new chemotherapeutic agents. The colchicine binding site is one of the most important pockets for potential tubulin polymerization destabilizers. Colchicine binding site inhibitors (CBSI) exert their biological effects by inhibiting tubulin assembly and suppressing microtubule formation. A large number of molecules interacting with the colchicine binding site have been designed and synthesized with significant structural diversity. CBSIs have been modified as to chemical structure as well as pharmacokinetic properties, and tested in order to find a highly potent, low toxicity agent for treatment of cancers. CBSIs are believed to act by a common mechanism via binding to the colchicine site on tubulin. The present review is a synopsis of compounds that have been reported in the past decade that have provided an increase in our understanding of the actions of CBSIs.

Characterization of the covalent binding of N-phenyl-N'-(2-chloroethyl)ureas to {beta}-tubulin: importance of Glu198 in microtubule stability

N-Phenyl-N'-(2-chloroethyl)ureas (CEUs) are antimicrotubule agents interacting covalently with β-tubulin near the colchicine-binding site (C-BS). Glutamyl 198 residue in β-tubulin (Glu198), which is adjacent to the C-BS behind the two potent nucleophilic residues, Cys239 and Cys354, has been shown to covalently react with 1-(2-chloroethyl)-3-(4-iodophenyl)urea (ICEU). By use of mass spectrometry, we have now identified residues in β-tubulin that have become modified irreversibly by 1-(2-chloroethyl)-3-[3-(5-hydroxypentyl)phenyl]urea (HPCEU), 1-[4-(3-hydroxy-4-methoxystyryl)phenyl]-3-(2-chloroethyl)urea (4ZCombCEU), and N,N'-ethylenebis(iodoacetamide) (EBI). The binding of HPCEU and 4ZCombCEU to β-tubulin resulted in the acylation of Glu198, a protein modification of uncommon occurrence in living cells. Prototypical CEUs then were used as molecular probes to assess, in mouse B16F0 and human MDA-MB-231 cells, the role of Glu198 in microtubule stability. For that purpose, we studied the effect of Glu198 modification by ICEU, HPCEU, and 4ZCombCEU on the acetylation of Lys40 on α-tubulin, a key indicator of microtubule stability. We show that modification of Glu198 by prototypical CEUs correlates with a decrease in Lys40 acetylation, as observed also with other microtubule depolymerizing agents. Therefore, CEU affects the stability and the dynamics of microtubule, likewise a E198G mutation, which is unusual for xenobiotics. We demonstrate for the first time that EBI forms an intramolecular cross-link between Cys239 and Cys354 of β-tubulin in living cells. This work establishes a novel basis for the development of future chemotherapeutic agents and provides a framework for the design of molecules useful for studying the role of Asp and Glu residues in the structure/function and the biological activity of several cellular proteins under physiological conditions.

Enantioselective total synthesis of otteliones A and B

Enantioselective total synthesis of otteliones A and B was accomplished. The key steps are radical cyclization of an alpha-iodoketone to construct the cis-hydrindanone skeleton and Suzuki-Miyaura coupling to incorporate the aromatic group. (+)-Ottelione A was converted to (-)-ottelione B on treatment with NaOH in THF.

Interaction of a cyclostreptin analogue with the microtubule taxoid site: the covalent reaction rapidly follows binding

The natural product cyclostreptin reacts covalently and stoichiometrically with microtubules, at either of two amino acid residues of beta-tubulin, Thr-218 or Asn-226, but much less extensively and only at Thr-218 in unpolymerized tubulin. It was found that 8-acetylcyclostreptin (8AcCS) induces tubulin assembly in a manner almost identical with that of cyclostreptin. We therefore synthesized [ (14)C-acetyl]8AcCS and studied the kinetics of its interaction with glutaraldehyde-stabilized microtubules and with unassembled tubulin. With the microtubules, we found that 8AcCS bound rapidly, with a minimal (unmeasurable with the radiolabeled analogue) lag prior to the occurrence of the covalent reaction. Apparent reaction rate constants for the overall reaction ranged from 6.2 x 10 (2) M (-1) s (-1) at 0 degrees C to 5.6 x 10 (3) M (-1) s (-1) at 20 degrees C. The rate constants obtained at 0 and 10 degrees C indicate an activation energy for the reaction of about 27 kcal/mol, while those obtained at 10 and 20 degrees C indicate an activation energy of about 7.7 kcal/mol. With the unpolymerized tubulin, we did find a minimal covalent reaction occurred without apparent microtubule assembly, but a substantial reaction only occurred following assembly. In conclusion, the radiolabeled 8AcCS shows that an extensive covalent interaction of ligand with tubulin requires microtubule assembly and that the covalent reaction occurs rapidly after the initial binding interaction.

Enantioselective Total Synthesis of (+)-Ottelione A, (−)-Ottelione B, (+)-3-epi-Ottelione A and Preliminary Evaluation of Their Antitumor Activity

Enantioselective total synthesis of (+)-ottelione A (1) and (-)-ottelione B (2), novel and potent antitumor agents from a freshwater plant, and (+)-3-epi-ottelione A (3), the earlier proposed stereostructure of 1, was efficiently achieved starting from the known tricyclic compound 10. The synthesis involved the following key steps: i) coupling reactions of aldehydes 8 and 9 with the aromatic portion 7 (8+7-->15 and 9+7-->27), ii) base-induced hemiacetal-opening/epimerization reactions of the cyclic hemiacetals 6 and 27 (6-->17 and 27 a-->26 a), and iii) Corey-Winter's reductive olefination of the cyclic thiocarbonates 21 and 36 (21-->22 and 36-->37). The present total synthesis fully established the absolute configuration of these natural products. The cell growth inhibition profile, COMPARE analysis, and tubulin inhibitory assay of (+)-3-epi-ottelione A (3) and its O-acetyl derivative 24 demonstrated that these unnatural substances could be prominent lead compounds for the development of anticancer agents with a novel mode of action.

Isoxazole-type derivatives related to combretastatin A-4, synthesis and biological evaluation

Novel combretastatin analogues bearing various five-membered heterocycles with consecutive oxygen and nitrogen atoms, in place of the olefinic bridge of CA4, have been synthesized (isoxazole, isoxazoline, oxadiazole, etc). These compounds have been evaluated for cytotoxicity and their ability to inhibit the tubulin assembly. On the basis of the relative position of the aromatic A- and B-rings on the heterocyclic moiety, they could be split in two classes, the alpha,gamma- or alpha,beta-diaryl heterocyclic derivatives. In the first series, the 3,5-diaryloxadiazole 9a displayed comparable antitubulin activity to that of CA4, but was devoid of cytotoxic effects. Among the alpha,beta-diaryl heterocyclic derivatives, the 4,5-diarylisoxazole 35 exhibited greater antitubulin activity than that of CA4 (0.75 vs 1.2 microM), but modest antiproliferative activity. These data showed that minor alteration in the chemical structure of the heterocyclic ring and its relative orientation with regard to the two phenyl rings of CA4 could dramatically influence the tubulin binding properties.

2-Cyano-3,12-dioxooleana-1,9(11)-diene-28-oic Acid Disrupts Microtubule Polymerization: A Possible Mechanism Contributing to Apoptosis

The semisynthetic triterpenoid 2-cyano-3,12-dioxooleana-1,9(11)-dien-28-oic acid (CDDO) has several biological activities, including the induction of apoptosis in many cancer cell lines. To identify potential protein targets, immobilized biotinylated CDDO was used to screen the proteome of a human lymphoma cell line (U937) sensitive to CDDO-induced apoptosis. Tubulin was identified as one of several putative targets of CDDO. CDDO was shown to selectively bind to tubulin, with a dissociation constant of approximately 7 microM, and to disrupt microtubules both in vivo and in vitro. CDDO inhibits tubulin polymerization in vitro, possibly through interactions with a hydrophobic site on beta-tubulin. The CDDO-tubulin interaction may also involve a reversible 1,4-addition with a protein sulfhydryl group. Unlike other known spindle poisons, CDDO does not result in a temporal increase in the mitotic index. Rather, CDDO seems to initiate apoptosis early in M phase.

Synthesis and biological evaluation of vinylogous combretastatin A-4 derivatives

Stereospecific syntheses of the Z-E and E-Z vinylogues of combretastatin A-4, and two B-ring related analogues, were achieved through a Suzuki-Miyaura coupling. As compared to CA4, the derivative with a phenyl moiety has shown increased potency in its ability to inhibit tubulin polymerisation.

Microtubule inhibitory effects of various SJ compounds on tissue culture cells

SJ compounds (SJ8002 and related compounds) are a group of novel anticancer agents (Cho, Chung, Lee, Kwon, Kang, Joo, and Oh. PCT/KR02/00392). To explore the anticancer mechanism of these compounds, we examined the effect of SJ8002 on microtubules of six human cell lines. At a high concentration (2 microg/mL), SJ8002 effectively disrupted microtubules of the six cell lines within 1 h. At lower concentrations (0.05 to approximately 1.0 microg/mL), the antimicrotubule activity of SJ8002 varied defending on cell lines. The inhibition of in vitro polymerization of pure tubulin by SJ8002 suggested that SJ8002 acts on free tubulin, inhibits the polymerization of tubulin dimer into microtubules, and hence induces the depolymerization of microtubules.

Total Synthesis and Absolute Configuration of Otteliones A and B, Novel and Potent Antitumor Agents from a Freshwater Plant

[reaction: see text] The first enantioselective total synthesis of otteliones A and B, biologically important and structurally novel natural products, has been successfully achieved. This total synthesis fully confirms the absolute configuration of these natural products.

Interactions of antimitotic peptides and depsipeptides with tubulin

Tubulin is the target for an ever increasing number of structurally unusual peptides and depsipeptides isolated from a wide range of organisms. Since tubulin is the subunit protein of microtubules, the compounds are usually potently toxic to mammalian cells. Without exception, these (depsi)peptides disrupt cellular microtubules and prevent spindle formation. This causes cells to accumulate at the G2/M phase of the cell cycle through inhibition of mitosis. In biochemical assays, the compounds inhibit microtubule assembly from tubulin and suppress microtubule dynamics at low concentrations. Most of the (depsi)peptides inhibit the binding of Catharanthus alkaloids to tubulin in a noncompetitive manner, GTP hydrolysis by tubulin, and nucleotide turnover at the exchangeable GTP site on beta-tubulin. In general, the (depsi)peptides induce the formation of tubulin oligomers of aberrant morphology. In all cases tubulin rings appear to be formed, but these rings differ in diameter, depending on the (depsi)peptide present during their formation.

Total synthesis of epi-otteliones

[reaction: see text] Syntheses of 6-epi- and 8-epi-otteliones, corresponding to earlier proposed structures of the biologically potent natural product ottelione A, have been accomplished from the readily available Diels-Alder adduct of cyclopentadiene and p-benzoquinone.

Involvement of microtubules and mitochondria in the antagonism of arsenic trioxide on paclitaxel-induced apoptosis

Arsenic trioxide (As(2)O(3)) at low concentrations (1-10 microM) is effective in the treatment of acute promyelocytic leukemia (APL) and lymphoma and is in clinical trials for treatment of solid tumors. Paclitaxel, an antimicrotubule agent, is highly efficacious in the treatment of adult tumors and is in clinical evaluation in childhood tumors. This study is the first to investigate the combination of arsenic and paclitaxel in the range of clinically achievable concentrations. We found that the simultaneous combination was antagonistic on proliferation of the neuroblastoma SK-N-SH cell line by using the combination index (CI) method. Moreover, a 40+/-5% decrease in paclitaxel-induced apoptosis in cells co-treated with As(2)O(3) confirmed the antagonism. The mechanism of antagonism was studied at the cellular level with 200 nM paclitaxel, twice the IC(50) value, and with 1 microM As(2)O(3) which administered singly did not affect cell survival or the microtubule network. As(2)O(3) antagonized the effects of paclitaxel on tubulin and microtubules. Paclitaxel-induced mitotic block was decreased by 20+/-2% and bundles induced by 200 nM paclitaxel were less condensed in the presence of 1 microM As(2)O(3). As(2)O(3) (10-200 microM) induced a concentration-dependent inhibition of tubulin polymerization in vitro which was maintained in presence of paclitaxel. Spectrophotometric and spectrofluorometric measurements indicated an interaction of As(2)O(3) with tubulin SH groups, without modification of the stoichiometry of paclitaxel binding to tubulin. Moreover, 4 microM As(2)O(3) inhibited the release of cytochrome c from isolated mitochondria by 78+/-10%. Our results show that As(2)O(3) and paclitaxel act antagonistically on mitochondria and microtubules and illustrate the need for careful evaluation of drug combinations.